What's the deal with Wellington saying Assaye was his greatest battle?

Well, it actually was his greatest battle, and in my opinion is that it's tied with Waterloo for his toughest challenge. Assaye was a hard battle, and Wellington (at the time Major General  Arthur Wellesley ) was vastly outnumbered. It was 9,500 to 70,800. Fortunately for Wellington, only 10,800 of the enemy under Anthony Pohlmann was a force to be reckoned with. The rest were an mass of undisciplined cavalry and infantry. To Pohlmann's advantage though he had more than 100 guns while Wellington only had 17. This looks like a set-up for a crushing defeat to the young Wellington. But through masterful strategic manoeuvres he was able to crush Pohlmann. Wellington had some 2,000 casualties to 6,000 on Pohlmann's end. The British were however unable to exploit this victory due to sheer exhaustion. This victory allowed the British to exploit more and more of India until it's independence in 1947. To read more see Assaye on Wikipedia. The battle honour is still carried by the Royal Highland Fusiliers (Formerly 74th Regiment), Light Dragoons (formerly 19th Light Dragoons), Madras Sappers (India, now repugnant),the Punjab Regiment of the Pakistani Army and the Seaforth Highlanders of the UK (not on colours).